A filter installation of conventional design in the art comprises an air inlet manifold, a battery of filter cells in parallel, each comprising one or more filter socks or the like, and a filtered air take-up manifold. For a suction type apparatus, air flow is achieved by a suction fan provided in the take-up manifold.
In this kind of apparatus, as the filters clog up, the filter capacity of the apparatus in terms of volume filtered per unit time is greatly reduced, and the apparatus must be unclogged from time to time.
To this end, it is conventional to provide controlled shutter means in each cell downstream from its filter sock, thereby enabling the outlet from the cell to be connected selectively either to the take-up manifold or else to ambient air at atmospheric pressure.
In filter mode, the cell is connected to the take-up manifold with the fan causing air to flow through the cell. In unclogging mode, the top portion of the cell under consideration is connected to ambient air, and since the pressure at the bottom of this cell is lower than atmospheric pressure (the filter system being a suction system) a backflow of air is established through the filter sock which is thus cleared off the particles that were previously adhering thereto. These particles then fall into the inlet manifold which has a conveyor device at the bottom thereof such as an Archimedes screw or the like for removing them.
Such a solution nevertheless suffers from drawbacks. Firstly when the particles coming off the filter during unclogging fall downwards, a substantial fraction thereof enter the inlet manifold to be refiltered by adjacent cells. Thus, in practice, these particles are not found in the dust-removal device but are to be found once again on filter membranes in adjacent cells which therefore clog up that much more quickly.
Secondly, it can happen in practice that the backflow velocity is too low to provide complete unclogging of the filter sock.
Batteries of filters are also known, in particular as described in German patent No. 897 044 and U.S. Pat. No. 3,951,623 in which backflow can be established individually in each filter for unclogging purposes by opening a valve provided between the inlet for air to be filtered and an auxiliary fan associated with a separator. The major drawback of such systems lies in that a large fraction of the air flowing to the auxiliary fan may come from the inlet for air to be filtered such that the flow rate of the backflow air passing through the filters is necessary limited and unclogging takes a long time or is incomplete.
German patent no. 925 392 describes a battery of filters in which the unclogging backflow is achieved by isolating the upstream end of each filter from the air inlet. However, in that device, as in the device described in above-mentioned German patent No. 897 044, the air leaving the auxiliary fan and the separator reenters the inlet for air to be filtered while it is still charged with fine particles. Thus, while the filter is being cleaned, a phenomenon occurs whereby the rate at which the other filters become clogged up is accelerated, as in the first-mentioned prior art.
The present invention seeks to mitigate the drawbacks of the prior art and, in a filter system of the type mentioned in the introduction, to provide much more effective means for unclogging each individual filter cell while also ensuring that no extra dust is added to the air being filtered by the adjacent cells.
Another object of this invention is to provide a filter system wherein means for generating the unclogging backflow, together with its auxiliary filter, can have reduced dimensions and power while still having a very effective unclogging action.